During the growth of citrus, the leaves appear chlorotic and yellow, and when they are severe, they die. The masses call this "yellow disease". The disease often leads to low citrus yields and poor quality, which is a major obstacle to citrus production. This article summarizes the causes of citrus "yellow disease" and the corresponding prevention measures as follows for your reference.
1. Nutritional disorders lead to yellowing of leaves
The citrus results are early, the result period is long, and the flowering amount is large. In one year, the spring, summer and autumn are generally taken three times. Winter warmth can also be used to warm the winter shoots, so there is no obvious deep dormancy period. Therefore, citrus needs to be hypertrophy, usually twice as many as deciduous fruit trees. The citrus roots are particularly developed, mainly distributed in 60 cm soil layers, especially in the 15 to 40 cm soil layer.
In the fruit-bearing period, 667 square meters of citrus yields 3,500 to 4,000 kilograms, and the absorption of nitrogen fN by the whole tree is 17.7 to 24.0 kilograms per 667 square meters. The absorption of phosphorus (P2O5) is 2.0 to 4.5 kilograms per 667 square meters. The absorption of (K2O) is 12.7 to 15.7 kg per 667 square meters, and the nutrient absorption ratio of nitrogen, phosphorus and potassium is 1:0.11 to 0.18:0.65 to 0.72.
1.1 deficiency yellowing
Deficiency is the most direct and sensitive factor causing yellowing of citrus leaves. There are many kinds of defects in leaf yellowing, such as nitrogen, potassium, calcium, magnesium, sulfur, boron, zinc, iron, molybdenum, manganese and other elements, which can make the leaves "yellow" and "yellow" The degree and characteristics of the difference are quite different.
1.1.1 Citrus nitrogen-deficient citrus yellow leaves are yellow and the leaves are evenly pale green. When the citrus is temporarily deficient in nitrogen, it is only expressed as pale green leaves. In the case of continuous nitrogen deficiency, the leaves are yellow and small, the branches are weak and slender, the growth of the branches is blocked, the branches are withered, and the yield is reduced. For the temporary nitrogen-deficient orange tree, foliar spray fertilizer can be used, generally 0.5% in spring and 0.3% urea liquid fertilizer in summer, spray once every 7 days, and spray 3 times. The fundamental measure to prevent continuous nitrogen deficiency is to increase soil fertility and apply nitrogen fertilizer on time and in volume.
1.1.2 citrus deficiency, potassium, citrus deficiency, potassium growth, weak growth. The leaves become smaller and turn yellow, and when the leaves are severe, the leaves shrink, and the edges of the green leaves are yellow, scorched, and the leaves are burning. The leaf surface can be uniformly sprayed with 0.3% to 0.6% potassium dihydrogen phosphate solution, and the effect is good.
1.1.3 The citrus-deficient spring leaves are yellowed at the top, and then expanded to the leaf margin. The leaf width of the diseased leaves is narrow and narrow, and the leaves of the yellowing disease fall off in advance. The deciduous branches often appear in the upper part of the crown. The physiological leaves are severely fruited and the fruit setting rate is low. In acidic soils, the calcium content is low: in warm and rainy areas, the calcium in the soil is easily leached and lost, resulting in insufficient calcium in the soil: excessive application of ammonia nitrogen fertilizer in the soil (such as ammonium sulfate, ammonium nitrate, etc.) Or the content of potassium and magnesium in the soil is too high, which causes imbalance of elements in the drought, and is easy to induce calcium deficiency. Apply lime to 100-150 kg / 667 square meters, adjust the soil pH value of 5.5 ~ 6.5 is appropriate.
1.1.4 Magnesium deficiency magnesium is the central element of chlorophyll. The yellow leaf of citrus deficiency magnesium appears. The leaf part of the diseased leaf and the middle vein is yellowed first, and the yellowing part is mostly rib-shaped. The tip and leaf base are often kept for a long time. Green. Acidic soil and light sandy soil are easy to lose magnesium, and excessive application of phosphorus and potassium is easy to induce magnesium deficiency. Foliar application of 0.3% to 0.5% of magnesium sulfate solution, spraying once every 7 days, even spraying 2 to 3 times can be effective.
1.1.5 Citrus sulphur-deficient and sulfur-deficient leaf color fades, showing light green or yellow-green, similar to the symptoms of nitrogen deficiency, easily confused with nitrogen deficiency, but most crops are more important than old leaves, while nitrogen deficiency is more important than old leaves. New leaf. The main vein of the diseased leaf is slightly yellower than other parts, especially the base of the main vein is yellower and easy to fall off. The new shoots are slender and mostly clustered. Application of sulfur-containing fertilizer gypsum, alum, sulfur, and ammonium sulfate, superphosphate, potassium magnesium sulfate are effective. Generally, the amount of pure sulfur (S) per hectare can be about 15 kg.
1.1.6 Citrus boron-deficient citrus requires large amount of boron, yellowing of boron-deficient leaves, yellowing of leaf and old leaves from veins, yellowing of whole leaves, thicker leafy flesh, more curling of leaf tips, and more veins The main and lateral veins are corked and cracked when severe. The young leaves produce an unshaped water-stained macula, which is distorted: some have water-stained black spots at the base of the main vein of the leaf back, which are easy to fall off. The tree is weak, the fruit is sparse, the fruit juice capsule shrinks and develops poorly, the slag is juicy, the fruit is brown and brown, and the flesh disappears when it is serious. The peel is thickened and shrunk, and the shape is small and hard as stone. ". Care should be taken to avoid excessive application of nitrogen, phosphorus, potassium, calcium and magnesium fertilizers. In acidic soils, it is not advisable to apply too much lime to avoid artificial boron deficiency. Control measures: (1) base application of boron fertilizer. The regular qualified boron fertilizer is directly scattered under the canopy, preferably covered with a thin layer of soil. The adult tree is applied with 15-20 g of boron fertilizer each time, and the light disease tree can be reduced. (2) Foliar spray of boron. Generally, the boron fertilizer (usually boric acid) is sprayed once in the early spring and full bloom, and the concentration is 0.2% to 0.3% in the early spring, and then 0.1% to 0.2%.
1.1.7 Citrus iron deficiency citrus is sensitive to iron deficiency. The leaves of the new shoots are chlorotic or yellowish, and the chlorosis is increased upwards in turn. The incidence of summer and autumn shoots is more than that of spring shoots. The leaves of severe yellow and white are burned, dried, and shed early, forming a tip or bald branch. Iron is not mobile in plants and is difficult to transport horizontally. On the same tree, the leaves on one branch are yellow and yellow, while the leaves on other branches remain green. Alkaline soil iron is not easily absorbed and utilized by plant roots; winter and spring drought, soil soluble iron content is reduced, resulting in insufficient iron rope: excessive phosphorus application in the soil is easy to induce iron deficiency. The lack of iron is difficult to solve by general fertilization because the effective ferrous iron (Fe2+) is easily oxidized to ferric iron (Fe2+) which is difficult to absorb and utilize. The spraying of 0.2% ferric citrate or ferrous sulfate has a partial effect. The yellow leaves of the leaves will produce more green small spots on the top of the leaves: it is best to spray the complex or chelated iron.
1.1.8 The leaves of citrus zinc deficiency new leaves are yellow or yellowish green, and the main and lateral veins and their adjacent tissues are still green. The main vein of the old leaf has an irregular green band, and the rest is pale green to orange-yellow. Some leaves show only yellow and yellowish spots between the main main veins of the green, and some of the leaves become smaller, narrower and chlorotic. When the zinc deficiency is severe, the apical shoot tips become shorter, the leaves are erect and narrow, and clusters of clustered lobes appear, and the plants are erect short clusters. Excessive application of nitrogen, phosphorus, potassium and calcium is easy to induce zinc deficiency. Foliar application of 0.5% zinc sulphate solution, spraying once every 7 to 10 days, continuous spraying 2 to 3 times, can also be combined with soil fertilization, 1 kg of zinc sulfate per 667 square meters. In addition, it can also be sprayed with 0.1% to 0.2% zinc oxide. For a slightly acidic soil (pH 5.5 to 6.0), a small amount of zinc sulfate can be applied to obtain good results, but the effect on alkaline soil is not obvious.
1.1.9 Citrus deficiency molybdenum mature leaves along the main vein local chlorosis and necrosis, namely citrus "macular disease". The new shoots are pre-existing, the young leaves are yellow-green, the veins are chlorotic, the new leaves are thinned, the leaves are deformed, and the growth is slow. The diseased leaves are curled to the front to form a cup, a tube or a whip, and in severe cases, the yellowing falls off. In the middle and lower leaves of the old branches, there are pale orange or yellow ovals, and the spots on the back of the leaves are brown. The diseased leaves are curled to the front to form a cup or a tube (called a cohesive disease). Molybdenum deficiency in acidic soils is more common. The amount of molybdenum fertilizer is very small, generally 30 to 60 grams per 667 square meters, generally not exceeding 150 grams / 667 square meters. The spraying of the leaves is mostly carried out, and the concentration of the ammonium molybdate or sodium molybdate solution is 0.05% to 0.1%.
1.1.10 The leaves of citrus-deficient young shoots are normal in size, yellow-green, green in the main and lateral veins and their adjacent tissues, and the rest are yellow-green, which is similar to the symptoms of zinc deficiency. However, the yellowing part of zinc deficiency is very yellow, the manganese deficiency is green, and the zinc deficiency young leaves are small and narrow. The size and shape of the leaves lacking manganese are basically normal, and the old leaves also show symptoms. Manganese is easily leached and lost in acidic soil: it is insoluble in alkaline soil and is not easily absorbed by plants: zinc deficiency, copper, magnesium and iron in soil can be accompanied by manganese deficiency. May to August can be sprayed with 0.6% manganese sulfate and 0.3 Baumeite sulphur mixture; or with 0.3% manganese sulfate plus equal amount of lime.
1.2 Excessive or improper fertilization causes yellowing
1.2.1 The tip of the chloride ion poisoning leaves begins to yellow, the yellow area expands downward, and then the tip of the leaf continues to yellow and decumulate, and the diseased leaves fall off in advance. It often causes a large number of leaves and fruits to fall off in the dry autumn, causing serious damage.
1.2.2 Excessive boron tip leaves are yellowed first, and spread along the leaf margin and midrib. Leaf tip and leaf edge become plaque, early defoliation, severe defoliation, dead branches, and finally the whole plant dies. After the yellow mottled part of the leaf tip of the old leaf, the mottled area is enlarged downward by the leaf tip along the leaf edge. Brown resin-like spots or mottled spots on the back of the leaf form irregular plaques. The irrigation water boron is >0.5 mm/kg, the soil water-soluble boron content is 1.58-1.79 mm/kg, and the boron content of the leaves exceeds 250 mm/kg.
1.2.3 Excessive copper occurs in the garden where the Bordeaux mixture is used too much. It shows a lot of fallen leaves. First, the old leaves fall off, and many small branches die, causing a reduction in production. Sometimes there are symptoms of iron deficiency and chlorosis. Or cause the bark to crack and flow, the roots of the roots are thickened and brown. Repeated spraying of Bordeaux mixture will slow down the growth of the tree body, and some leaves will appear, the fruit will become smaller and the yield will decrease. In severe cases, a large number of fallen leaves, withered branches, and even the death of the whole tree.
1.2.4 Excessive yellow mottled at the tip and leaf edge of the blade of sulfur, similar to the excess of boron, then the yellow area expands and extends inward, while the main vein remains green for a long time. In severe cases, the whole leaf yellow falls off early, and the branch tip Withered, the whole tree is dead. When nitrate is added, the number of yellow leaves and mottles is reduced, but not completely eliminated.
1.2.5 Application of unfertilized organic fertilizer causes yellow leaves, such as organic fertilizers such as chicken manure that have not been fermented and decomposed, and organic fertilizer such as chicken manure will ferment in the underground when it encounters suitable temperature and humidity. The release of heat, causing damage to the underground hair roots, hindering the timely supply of nutrients and moisture in the ground, causing yellowing of the underlying and above-ground nutrients. Therefore, when applying organic manure such as chicken manure or pig manure as the base fertilizer, it must be fermented and fermented before application.
Causes of citrus yellowing and corresponding control measures
2. Environmental climatic conditions cause yellowing of citrus leaves
2.1 Soil texture sticky weight caused by yellow leaves
Under the condition of texture stickiness and soil compaction, the ventilation is not smooth, which affects the gas exchange and absorption on the root surface, and the root activity is obviously weakened. It hinders the absorption and utilization of nutrients by roots, which causes the leaves to lose green and yellow.
2.2 Soil moisture imbalance causes yellow leaves
In the case of too low soil moisture and very dry conditions, soil nutrients can not move with water, spread to the surface of the roots for crops to absorb and use; excessive water, such as too much precipitation or excessive irrigation, the soil air is significantly reduced, forming anoxic The condition affects root respiration and weakens the absorption and utilization of nutrients, water and gases in the roots, causing yellowing of the leaves.
2.3 freezing damage caused by yellow leaves
When the temperature is too low, the soil nutrients are not easily activated or released, and the crop is difficult to absorb. Lead to spring germination and yellow leaves.
2.4 excessive ringing caused by yellow leaves
Ring stripping should be based on the strength of the tree. If the ring is peeled too much, the bark wounds heal slowly, resulting in the imbalance of the nutrients and water in the lower part of the tree and the aboveground parts, and the yellowing of the leaves occurs. This can be stripped with a clean plastic cloth wrap ring and sprayed with urea and potassium dihydrogen phosphate foliar fertilizer on the leaves of the tree.
2.5 herbicide phytotoxicity caused by yellow leaves
The herbicidal mechanism of various herbicides is different. If the herbicide is not used blindly according to the instructions or if the herbicide is applied in excess, the herbicide will be harmed and the yellow leaves will be caused. If there is no trace, it can only exert contact and systemic killing effect on the aboveground green plant tissue, so the impact on the underground root system of fruit trees is also small. Glyphosate is a herbicide herbicide that rots whole weeds, including ground green leaves and underground roots, by inducing conduction to the green tissue on the ground. Its impact on fruit trees is enormous. If glyphosate is used multiple times within one year, the underground roots of the fruit trees will inevitably cause damage, resulting in malnutrition in the aboveground parts. Yellow leaves appear. Therefore, when using herbicides to control weeds, we must first understand the herbicidal mechanism of various herbicides, and then select them according to the actual situation, and spray with appropriate concentration of herbicides to avoid unnecessary economic losses.
3, pests and diseases cause yellowing of citrus leaves
Through the invasion of pathogens such as pathogens, viruses and mycoplasma, the root tissue and leaf tissue of the fruit trees can be destroyed. Such as root rot, rhizomes and other diseases are most common and serious, because the various secretions produced can block the transport system, so that it can not absorb water and nutrients, the raw materials necessary for photosynthesis are not available, and the leaves are lost. Green yellowing: Sometimes the pathogen directly invades the leaves, destroys the chlorophyll, directly reduces the photosynthesis ability of the leaves, and causes the leaves to rapidly develop "yellow disease".
Red spider, yellow spider, scale insect, and leafhopper are mainly harmful to citrus leaves, shoots and fruits. When the leaves are damaged, the leaves become grayish white and lose their luster until they are yellow and fall off. Star snails, brown beetles, worms and worms mainly harm citrus roots, trunks and main branches, causing damage to the channels that absorb and transport nutrients in roots, trunks, etc., causing branches of yellow leaves and even whole fruit trees. Withered.
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